The present invention relates to a method of driving a planar display device of the so-called "active matrix" type, i.e. a display device comprising a set of row conductors and a set of column conductors, and an array of picture elements (pixels) each made up of a display element such as a liquid crystal display element coupled to a switching element such as a transistor, each picture element being disposed at an intersection of a row and a column conductor, and with each switching element acting to control the transfer of a video data signal to be stored as a charge in a capacitance coupled to the corresponding display element, with the amount of charge determining the optical state of that display element. This capacitance can be the self-capacitance of the display element, or an additional small capacitor coupled thereto. In particular, the present invention is directed towards a drive method for a display device which is employed for applications such as television image display, whereby successive rows of picture elements are sequentially scanned to be updated in accordance with the contents of a video signal, with all of the rows being scanned during a periodically repeated interval generally referred to as a field. That is to say, a complete image is generated by the display during each field.
Such active matrix display devices have not yet reached the stage of practical mass-production manufacture, although they possess significant advantages over simpler types of matrix display device (i.e. those which do not incorporate an individual switching element to control each display element in the matrix) with regard to achieving a high level of contrast even when the number of elements in the display is very large. The major factor which has prevented the practical application of such active matrix display devices lies in the manufacturing difficulties which result when it is attempted to provide switching elements in the display matrix which will have good electrical characteristics, i.e. which will have a low level of leakage current. Due to the very large number of these switching elements necessary in a matrix, and particularly in the case of a matrix having a very large number of picture elements such as is required for television display, and due to the fact that each switching element must generally be of very small size in order to avoid excessive reduction of the effective display area, the manufacturing costs tend to be extremely high, and the manufacturing yield low. Non-linear resistance elements (generally referred to as NLREs) have been proposed for use as switching elements to enable low-cost active matrix display devices to be manufactured, due to the production process required for these being less complex than that for thin-film transistors used as switching elements. However such nlres result in higher levels of leakage current than do thin-film transistors, so that these have not so far been practically implemented into matrix display devices.
There is therefore an urgent requirement for some means of relaxing the requirements set upon the switching elements of such display devices and in particular for means to enable switching elements having a relatively high leakage current to be employed, both to enable simpler and more easily manufactured elements to be used and also to enable the manufacturing yield to be increased. This requirement is met by the drive method of the present invention.